Core Data 改革：实现 SwiftData 般的优雅并发操作

东坡肘子

发表于 2024 年 4 月 18 日

@ModelActor
actor DataHandler {
    func updateItem(identifier: PersistentIdentifier, timestamp: Date) throws {
        guard let item = self[identifier, as: Item.self] else {
            throw MyError.objectNotExist
        }
        item.timestamp = timestamp
        try modelContext.save()
    }
}

public final class CustomExecutor: SerialExecutor {
  // 串行工具
  private let serialQueue: DispatchQueue
  public init(serialQueue: DispatchQueue) {
    self.serialQueue = serialQueue
  }

  public func enqueue(_ job: consuming ExecutorJob) {
    // 转换 ExecutorJob 为 UnownedJob
    let unownedJob = UnownedJob(job)
    let unownedExecutor = asUnownedSerialExecutor()
    // 在串行队列中执行任务
    serialQueue.async {
      unownedJob.runSynchronously(on: unownedExecutor)
    }
  }
 
  // 转换自身为 UnownedSerialExecutor
  public func asUnownedSerialExecutor() -> UnownedSerialExecutor {
    UnownedSerialExecutor(ordinary: self)
  }
}

public final class NSModelObjectContextExecutor: @unchecked Sendable, SerialExecutor {
  public final let context: NSManagedObjectContext
  public init(context: NSManagedObjectContext) {
    self.context = context
  }

  public func enqueue(_ job: consuming ExecutorJob) {
    let unownedJob = UnownedJob(job)
    let unownedExecutor = asUnownedSerialExecutor()
    context.perform {
      unownedJob.runSynchronously(on: unownedExecutor)
    }
  }

  public func asUnownedSerialExecutor() -> UnownedSerialExecutor {
    UnownedSerialExecutor(ordinary: self)
  }
}

public nonisolated var unownedExecutor: UnownedSerialExecutor

actor DataHandler {
  public nonisolated let modelExecutor: CoreDataEvolution.NSModelObjectContextExecutor
  public nonisolated let modelContainer: CoreData.NSPersistentContainer

  public init(container: CoreData.NSPersistentContainer) {
    // 初始化私有上下文
    let context = container.newBackgroundContext()
    // 实例化自定义执行者
    modelExecutor = CoreDataEvolution.NSModelObjectContextExecutor(context: context)
    modelContainer = container
  }
  
  // 获取 Actor 所需的自定义执行者（UnownedSerialExecutor）
  public nonisolated var unownedExecutor: UnownedSerialExecutor {
    modelExecutor.asUnownedSerialExecutor()
  }

  // 用于 Actor 中数据操作的托管对象上下文
  public var modelContext: NSManagedObjectContext {
    modelExecutor.context
  }

  // 实现类似于 SwiftData 的托管对象访问机制
  public subscript<T>(id: NSManagedObjectID, as _: T.Type) -> T? where T: NSManagedObject {
    try? modelContext.existingObject(with: id) as? T
  }
}

public protocol NSModelActor: Actor {
  /// 为 NSModelActor 指定的 NSPersistentContainer
  nonisolated var modelContainer: NSPersistentContainer { get }

  /// 协调模型 actor 访问的执行者。
  nonisolated var modelExecutor: NSModelObjectContextExecutor { get }
}

extension NSModelActor {
  /// 模型 actor 的执行者的优化、非拥有引用。
  public nonisolated var unownedExecutor: UnownedSerialExecutor {
    modelExecutor.asUnownedSerialExecutor()
  }

  /// 序列化运行在模型 actor 上代码的上下文。
  public var modelContext: NSManagedObjectContext {
    modelExecutor.context
  }

  /// 根据指定的标识符返回模型，向下转型为适当的类。
  public subscript<T>(id: NSManagedObjectID, as _: T.Type) -> T? where T: NSManagedObject {
    try? modelContext.existingObject(with: id) as? T
  }
}

@attached(member, names: named(modelExecutor), named(modelContainer), named(init))
@attached(extension, conformances: NSModelActor)
public macro NSModelActor() = #externalMacro(module: "CoreDataEvolutionMacrosPlugin", type: "NSModelActorMacro")

public enum NSModelActorMacro {}

extension NSModelActorMacro: ExtensionMacro {
  public static func expansion(of _: SwiftSyntax.AttributeSyntax, attachedTo _: some SwiftSyntax.DeclGroupSyntax, providingExtensionsOf type: some SwiftSyntax.TypeSyntaxProtocol, conformingTo _: [SwiftSyntax.TypeSyntax], in _: some SwiftSyntaxMacros.MacroExpansionContext) throws -> [SwiftSyntax.ExtensionDeclSyntax] {
    // 生成符合 NSModelActor 协议的扩展代码
    let decl: DeclSyntax =
      """
      extension \(type.trimmed): CoreDataEvolution.NSModelActor {}
      """

    guard let extensionDecl = decl.as(ExtensionDeclSyntax.self) else {
      return []
    }

    return [extensionDecl]
  }
}

extension NSModelActorMacro: MemberMacro {
  public static func expansion(of _: AttributeSyntax, providingMembersOf _: some DeclGroupSyntax, conformingTo _: [TypeSyntax], in _: some MacroExpansionContext) throws -> [DeclSyntax] {
    // 添加构造器及所需属性
    [
      """
      public nonisolated let modelExecutor: CoreDataEvolution.NSModelObjectContextExecutor
      public nonisolated let modelContainer: CoreData.NSPersistentContainer

      public init(container: CoreData.NSPersistentContainer) {
        let context = container.newBackgroundContext()
        modelExecutor = CoreDataEvolution.NSModelObjectContextExecutor(context: context)
        modelContainer = container
      }
      """,
    ]
  }
}